Covering elements comprising (i) an organic matrix and (ii) luminescent doping materials, scintillating doping materials or mixtures thereof, which form at least one light cascade absorbing electromagnetic radiation in at least one first predetermined wavelength band in order to re-emit the same in at least one second predetermined wavelength band are known for use in agricultural greenhouses and as herbicidal films (French patent application Ser. No. 8,116,678, filed Sept. 2, 1981, which corresponds to French Patent Publication No. 2,511,840 and French patent application Ser. No. 8,508,385, filed June 4, 1985, which corresponds to U.S. patent application Ser. No. 07/022,640, filed Feb. 4, 1987 now abandoned.
French patent application Ser. No. 8,116,678 filed Sept. 2, 1981, which corresponds to French Patent Publication No. 2,511,840, teaches covering elements useful in agricultural greenhouses which absorb the wavelength bands which are detrimental to the development of plants while at the same time re-emit wavelength bands which promote the photosynthesis of the plants.
On the other hand, French patent application Ser. No. 8,508,385, filed June 4, 1985, which corresponds to U.S. patent application Ser. No. 07/022,640, filed Feb. 4, 1987, teaches covering elements useful as herbicidal films which absorb the wavelength bands which are favorable to the photosynthesis of plants while at the same time re-emit wavelength bands which are detrimental to the development of the plants.
The above two covering elements have provided excellent results during controlled experiments and use in various climates. However, these covering elements have two major disadvantages. First, the doping materials used to form the light cascades are very expensive. Second, the stability of the doping materials in the organic matrix is relatively limited. This considerably reduces the effective life span of the covering elements, which is a major disadvantage for agricultural applications.